Nov 28 (Reuters) - Energy Transfer Partners LP sought permission from U.S. regulators on Tuesday to start horizontal drilling at six sites in Ohio and West Virginia as it works to complete part of its Rover natural gas pipeline by year-end.

The request came a few days after environmental regulators in Ohio told ETP to “pause horizontal drilling activities” after a recent fluid spill.

The Ohio Environmental Protection Agency on Nov. 24 cited Rover for spilling contaminants into the Black Fork of the Mohican River in Ashland County in the north central part of the state.

Officials at ETP were not immediately available for comment.

Pipeline companies use horizontal drilling to cross under large obstacles like highways and rivers.

Before the pause by the Ohio EPA, ETP said it expected to complete the first phase of the project in Ohio, Pennsylvania and West Virginia by the end of the year and the second phase from Ohio to Ontario by the end of the first quarter of 2018.

Once complete, the $4.2 billion Rover will carry up to 3.25 billion cubic feet per day of gas from the Marcellus and Utica shale fields in Pennsylvania, Ohio and West Virginia to the U.S. Midwest and Ontario in Canada. One billion cubic foot per day of gas is enough for about 5 million U.S. homes.

Ohio EPA said this was Rover’s fifth notice of violation since it received permission from the U.S. Federal Energy Regulatory Commission (FERC) in September to resume horizontal drilling.

FERC banned ETP from new horizontal drilling in May after the company spilled around 2 million gallons of drilling fluid into the Tuscarawas River wetland in Ohio, among other things.

FERC’s Office of Enforcement is investigating that spill in part because the fluid released contained traces of diesel fuel.

Ohio EPA said the latest incident at the Mohican River was Rover’s 19th notice of environmental violation in the state this year. ETP released 200 gallons of drilling fluid - a clay and water mix - into a tributary of the Mohican River, the Ohio EPA said.

Ohio EPA also said Rover is in violation of a July 7 order that required the company to file for a construction storm water permit. The EPA said it has referred the case to the Ohio attorney general since Rover has refused to comply with the order or pay an appropriate civil penalty.